An OFDM system is a frequency-division multiplexing (FDM) system using digital multi-carrier modulation. A plurality of orthogonal sub-carriers is used to transfer data. The data are divided into a plurality of parallel data streams or channels corresponding to the sub-carriers respectively. Each sub-carrier carries a single symbol, each symbol corresponds to several sets of bits, and each symbol of 16 quadrature amplitude modulation (QAM) corresponds to 4 bits. Corresponding manners include, for example, QAM or phase shift modulation, which is also referred to as phase shift keying (PSK).
The OFDM technology is usually applied in the field of wireless communication, which possibly generates a multi-path effect. The multi-path effect may cause time-spreading and inter-symbol interference (ISI). This is a so-called frequency-selective channel. The frequency-selectivity is generally overcome by adding a guard interval in each OFDM symbol.
When a transmitter and a receiver of the broadband mobile communication applying an OFDM technology are respectively located in a base station and a vehicle moving at a high speed (for example, an express train), relative movement there-between may generate Doppler Effect. The Doppler Effect may cause a channel response in a transferred OFDM symbol to become time-varying. That is a so-called time-selective channel. Orthogonality between the sub-carriers is further damaged, which is called an inter-carrier interference (ICI). The faster the vehicle moves, the more apparent the ICI will be.
A common approach for eliminating the ISI is to add a guard interval in each OFDM symbol, which may prolong a symbol period. However, the longer the symbol period is, the greater the influences of the Doppler Effect will be. Therefore, in order to eliminate the wireless channel time-varying and frequency-selective effects that the broadband mobile communication encounters during a high speed movement, a channel estimation technology is usually utilized.
In addition, the channel estimation technology may also be obtained with reference to the following documents:
(i) an article published by S. Chen and T. Yao (S. Chen and T. Yao, “Intercarrier interference suppression and channel estimation for OFDM systems in time-varying frequency selective fading channels,” IEEE Trans. Consum. Electron., vol. 50, no. 2, pp. 429-435, May 2004.);
(ii) an article published by Y. Mostofi and D. C. Cox (Y. Mostofi and D. C. Cox, “ICI mitigation for pilot-aided OFDM mobile systems,” IEEE Trans. Wireless Commun., vol. 4, no. 2, pp. 765-774, March 2005);
(iii) an article published by H. S. Cho (H. S. Cho, “Midamble aided OFDM performance analysis in high mobility vehicular channel,” 802.11 WLAN WG, Jan. 14, 2008); and
(iv) an article published by H. C. Lee, C. W. Chen, S. M. Young, and Shyue-Win Wei (H. C. Lee, C. W. Chen, S. M. Young, and Shyue-Win Wei, “Matrix Channel Estimation for OFDM Systems with Two Training Symbols and High-Order Polynomial Fitting,” In Proc. 18th Annu. IEEE Int'l Symp. on Personal, Indoor and Mobile Radio Communications, Athens, Greece, September 2007, pp. 1-5).
The accuracy of time-varying channel estimation may influence performance of an OFDM frequency domain equalizer or signal detection performance, which further influences the accuracy for demodulating transferred signals. Therefore, the quality of the channel estimation technology directly influences performance of the whole OFDM system.